1. Field of the Invention
The present invention relates in general to techniques associated with a bonded composite disk such as a digital video disk (DVD), which is produced by bonding together two separately formed disk components. More particularly, this invention is concerned with an improved structure of a bonded composite disk of the type in which only one of the two disk components has an information-bearing surface while the other disk component serves as a dummy disk component which does not have an information-bearing surface. This type of bonded composite disk is referred to as a bonded composite disk of single-side recorded type. This invention also concerns with improvements of a method of producing the bonded composite disk, and a mold for forming the two separate disk components that are bonded together to produce the bonded composite disk.
2. Discussing of the Related Art
A bonded composite disk consisting of two separately formed disk components that are bonded together has recently been developed in an effort to provide a disk having increased data storage capacity. Such a bonded composite disk finds industrial applications as a digital video disk (DVD), for example. This bonded composite disk is capable of storing a large volume of information, for example, information for a long film or movie, on two information-bearing surfaces thereof which are formed on respective two separately formed disk components. The bonded composite disk may be used to store an ordinary amount of information, for example, information for an ordinary film. In this case, only one of the information-bearing surfaces is used to store the data. Namely, the bonded composite disk is usable as the composite disk of single-side recorded type. The bonded composite disk of a single-side recorded type consists of a first disk component having an information-bearing surface on one of its opposite surfaces and a second disk component without an information-bearing surface. The first and second disk components are superposed on each other at their information-bearing surfaces. The bonded composite disk of single-side recorded type is usually produced by forming the first and second disk components of a resin material using a mold as used for forming an ordinary disk, such that each of the formed first and second disk components has a thickness slightly smaller than a half of the nominal thickness of the composite disk to be produced. A reflector film and a protective film are formed on the information-bearing surface of the first disk component. Then, the second disk component is superposed on the information-bearing surface of the first disk component with a layer of a bonding adhesive applied therebetween, so that the disk components are bonded together into the bonded composite disk of single-side recorded type.
Such an adhesive layer interposed between the first and second disk components may advantageously function to accommodate a thickness variation of the bonded composite disk from its nominal value, if the thickness of the adhesive layer is sufficiently large. In some cases, however, the thickness of the adhesive layer cannot be made sufficiently large due to particular properties of the adhesive material and/or required dimensional specifications of the bonded composite disk. For example, where a digital video disk (DVD) is produced using a bonding adhesive which is curable by exposure to a ultraviolet radiation (UV-curable adhesive), it is generally difficult to form the adhesive layer with a thickness larger than 40 .mu.m due to the properties of the UV-curable adhesive, for example, a photosensitive polymer.
Although the adhesive layer having a relatively small thickness may satisfy the requirements of the first and second disk components in terms of their configuration and dimensional accuracy, the small thickness of the adhesive layer may cause poor bonding and low thickness accuracy of the bonded composite disk consisting of the first and second disk components. Thus, the prior art disk suffers from a problem of a relatively high ratio of rejects. Although poor bonding and low thickness accuracy of the bonded composite disk do not cause a serious problem in the research and development stage of such bonded composite disk, they are considered to be a considerable barrier to achieve adequate production of the bonded composite disk on a commercial basis.